By Marc Lourdes and Joshua Berlinger CNN

HONG KONG (CNN) -- North Korean leader Kim Jong Un's half brother died Monday after becoming ill at Kuala Lumpur International Airport in Malaysia, according to Selangor State Criminal Investigations Department Chief Fadzil Ahmat.

Another police source told CNN the man who died is named Kim Jong Nam.

Police have described the incident as a "sudden death," pending the results of a postmortem investigation. There are unsubstantiated reports circulating that Kim Jong Nam may have been attacked.

"So far there are no suspects, but we have started investigations and are looking at a few possibilities to get leads," Fadzil told Reuters.

"The deceased ... felt like someone grabbed or held his face from behind," Fadzil said. "He felt dizzy, so he asked for help at the ... counter of KLIA."

South Korean media is reporting that he was in his 40s.

The Malaysian inspector general of police said in a news release the deceased North Korean man was traveling with a passport bearing the name Kim Chol.

Kim Jong Nam was the most public of all of Kim Jong Il's sons before Kim Jong Un took power, though most remember him for the scandalous headlines he made.

Reports surfaced that Kim Jong Nam fell from favor from the ruling family after he was caught trying to sneak a visit to Tokyo Disneyland using a forged document in 2001 -- an assertion that one journalist who spoke to Kim countered in a 2012 book.

It is not clear when he left North Korea, but Kim Jong Nam spent time living in Macau and China and was absent from his father's funeral in 2011, fueling earlier rumors that he had been banished from the country.

Kim was an overweight and careless playboy, but also a smart and open-minded man who was willing to speak out against the family, according to Yoji Gomi, the author of the 2012 book "My Father, Kim Jong Il, and Me."

"He spoke out against his father's 'military first' policy," Gomi told CNN in 2012. "He wants North Korea to embrace economic reform and open its doors."

Kim Jong Nam believed his youngest brother, Kim Jong Un would fail as North Korea's leader, according to Gomi -- though the two brothers never met because of the ancient practice of raising potential successors separately.

A third sibling, Kim Jong Chul, was rumored to have been ruled out as his father's successor for being too "effeminate," Scott Snyder of the Council on Foreign Relations wrote in 2012.

Leadership purges

The eldest Kim brother's death comes as Kim Jong Un has purged senior leadership around him, including firing the minister of state security -- one of his top lieutenants -- earlier this month.

A report published by a South Korean think tank at the end of December claimed that Kim had ordered the execution of 340 people since coming to power.

The news of Kim Jong Nam's death broke two days after Pyongyang conducted its first ballistic missile test of 2017.

North Korean state media reported that Kim Jong Un oversaw the test of its new, nuclear capable Pukguksong-2 missile, which it described as successful.

Two US officials said the launch was the first land-based test of an intermediate-range missile. In the past, has been fired from a submarine.

CNN's Andreena Narayan, Sandi Sidhu and journalist KL Chan contributed to this report

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